The kind of fuel I use has nothing to do with how to inspect a
furnace filter. The topic to which you replied was specifically
about how to inspect furnace filters. Look at the subject line
above. When you asked I told you and then wanted to know what
it had to do with inspecting furnace filters The answer is
nothing. If I use dried manure to warm the house it still
doesn't have anything to do with how to inspect the filters.
The question was not how fast the filters get dirty. The question is
how to inspect them to know when they are dirty enough to be changed.
This has nothing to do with how much dirt there is in the air. If
there is a lot they will have to be replaced oftener than if there
isn't. But the inspection procedure will be the same.
Is there some reason that you insist that you know better than I do
what my question was? It seems more like the activity of a troll
than a desire to help others. I give people a chance to show that
they are really interested in helping. But I don't do it forever
when they refuse to actually address the question.
On Tuesday, October 6, 2015 at 6:33:57 PM UTC-4, Bill Gill wrote:
Seems to me the question comes down to two choices.
1. Is it a push or a pull?
2. What are you going to check?
Push or pull means do I have to remember to go look at it and pull the info
rmation, or does it push it to my attention somehow?
What are you going to check - really just two obvious choices: visual appe
arance of dirt, or effect on the airflow. Effect on the airflow can be mea
sured by the noise (air passing through, or some filters have a whistle bui
lt in) or by pressure differential (liquid or other manometer). You can se
t up a manometer to give you an alarm, in which case it is a push, or you c
an remember to go look at it periodically.
I am visually inspecting it periodically. I haven't been able to
find an off-the-shelf pressure sensor that I can use. I recall
from 50 or 60 years ago seeing advertised a sensor that switched
over from white to red when the filter needed to be replaced, but
I haven't seen anything like that in many years. As far as a whistle
is concerned I probably wouldn't be able to hear it. My high
frequency hearing is poor. That would also probably be in high
priced filters. I prefer the lowest priced ones that will do the
job. What I have been trying to get guidance on is what are the
criteria that would mean I needed to replace the filter.
On Thursday, October 8, 2015 at 9:08:50 AM UTC-4, Bill Gill wrote:
information, or does it push it to my attention somehow?
appearance of dirt, or effect on the airflow. Effect on the airflow can be
measured by the noise (air passing through, or some filters have a whistle
built in) or by pressure differential (liquid or other manometer). You ca
n set up a manometer to give you an alarm, in which case it is a push, or y
ou can remember to go look at it periodically.
They start to look dirty compared to a new one, works for me.
Don't be a tight ass and just buy the inexpensive ones and change them out
every month or two. You are only talking less than $ 15 per year.
I have been doing that for years. I did have a man out after 8 years on a
new unit becuse the heat pump quit. Bad starting capacitor. He cleaned the
coils and said the inside ones did not need any cleaning, but did it anyway
as it was part of the service.
We do not open any windows so the inside air tends to stay clean.
We do it monthly as it's easier to just *remember*, come the first of
each month, to remove one of the NEW filters from the bag that is kept
near the furnace and replace the one *in* the furnace.
OTOH, if we had purchased the fancy shmancy HEPA filters, we'd
be less inclined to do so as they are pricey and, according to
the sales folks, don't really make much of a PRACTICAL difference.
We had the ducts cleaned when we bought the house many years (decades)
ago and they are still clean. So, likely that the filters are
doing their job.
On Thursday, October 8, 2015 at 10:24:17 AM UTC-4, Don Y wrote:
I find it rather fascinating that you change filters every month
but believe some salesman that a better filter doesn't make a
difference. Also, AFAIK, there isn't really a HEPA filter for
furnaces, but there are lots of MERV filters that are way better than
the 1" thick cheap ones. Those trap so little that I can't imagine
how they could need changing in only a month, unless you're living
in a dust storm. And those are there mostly to try to protect the eqpt
from excessive crap, not to remove particulates from the air for
On Thursday, October 8, 2015 at 10:24:17 AM UTC-4, Don Y wrote:
Okay, think about WHY you change the filter at all.
Does it filter more poorly as it gets dirty?
No, of course not. The dirt is on the side the air goes in. The more dirt
there, the more efficiently it traps even more dirt. It doesn't start let
ting more through. Dirty filters are better, UNTIL:
Dirty filters trap more dust, but they also have more resistance to the air
flow. Usually there is plenty of air flow to spare, or those 4 inch filte
rs wouldn't work. But it does make the fan work harder, if it has some way
to keep the air flow constant. If the filter is really dirty, it could th
eoretically slow down the air to where you wouldn't get cooling or heating.
Then if the filter becomes completely blocked, sometimes it will rip and th
en unfiltered air can hit your coils.
At any rate, changing too often is a bad idea.
Are you joking or what? As you mentioned when air flow decreases heating
efficiency drops which will increase your energy bill. How about your
car air filter? Dirty filter is bad, drop in MPG, hard on the engine not
breathing well, etc. I replace filter every 6 months. Merv 10 rated 16 x
25 x 5" pleated one.
On Friday, October 9, 2015 at 6:39:45 PM UTC-4, Tony Hwang wrote:
dirt there, the more efficiently it traps even more dirt. It doesn't start
letting more through. Dirty filters are better, UNTIL:
air flow. Usually there is plenty of air flow to spare, or those 4 inch f
ilters wouldn't work. But it does make the fan work harder, if it has some
way to keep the air flow constant. If the filter is really dirty, it coul
d theoretically slow down the air to where you wouldn't get cooling or heat
On Friday, October 9, 2015 at 3:13:47 PM UTC-4, TimR wrote:
rt there, the more efficiently it traps even more dirt. It doesn't start l
etting more through.
OK, so if it traps more "even more dirt" as it gets dirtier, then, by defau
it must be letting some dirt through while clean, right? If not, where did
that "even more dirt" that it is now trapping go while the filter was clean
The air in the house didn't get dirtier as the filter got dirtier, so based
on your theory, all of that "even more dirt" must have gone through the fil
until it got dirty enough to trap it.
Based on your theory, shouldn't we take our clean filters and load them up
with dirt before we install them so they'll trap all that "even more dirt"
right from the start.
ir flow. Usually there is plenty of air flow to spare, or those 4 inch fil
ters wouldn't work. But it does make the fan work harder, if it has some w
ay to keep the air flow constant. If the filter is really dirty, it could
theoretically slow down the air to where you wouldn't get cooling or heatin
On Friday, October 9, 2015 at 11:21:39 PM UTC-4, DerbyDad03 wrote:
That seems reasonable. Furnace filters are rated by how small a
particle they will trap. Smaller than the rating, they go through.
But over time, as the filter gets more plugged up with particles,
it seems likely that it will trap start trapping the finer particles.
He did point out the other important side of that equation, which
is that as the filter plugs up with dirt, the resistance to airflow
goes up and the closer it gets to needing to be replaced. So, I think
if you pre-loaded a new filter with some dirt, instead of having a
filter that will trap X micron particles, you'd have a filter that would
trap some of the smaller particles too, but airflow would be less and
the filter would be Y months closer to needing to be replaced.
That's why if you want a filter that traps smaller particles, they
have them and they are 5" thick and cost a lot more money.
You could always build a bracket that somehow would fit in the available op
ening, but permit you to use a smaller filter that was thicker. But, you h
ave to be careful not to restrict the airflow very much as the furnaces ar
e designed to be at their maximum efficiency based on a certain airflow. A
lso, too low a flow could also lead to blower motor overheating.
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